The present invention relates to cationic catalysts for the treatment of a polysilazane and, more particularly, to a cationic catalysis process for the treatment of a polysilazane containing on average at least two .ident.SiH groups per molecule.
Organopolysilazanes, referred to as polysilazanes hereinafter, are well-known products that occur in the form of monomers, oligomers, ring or linear polymers, and resinous polymers. These polysilazanes an be prepared using a wide variety of methods from a wide range of starting materials.
Particularly, these polysilazanes can be shaped and pyrolyzed in the form of Si.sub.3 N.sub.4, SiC or their mixtures. The silicon nitride can also be manufactured by reacting chlorosilanes, such as SiCl.sub.4, HSiCl.sub.3, and H.sub.2 SiCl.sub.2, with ammonia at a high temperature and in a vapor phase. It is difficult to manufacture shaped articles, especially fibers, because this method directly yields a powered product. In contrast, polysilazanes can also be spun into continuous fibers that yield ceramic fibers upon pyrolysis.
The polysilazanes can be shaped into films of various gauges and massive molded articles. They can also be used as a binder for ceramic fiber or carbon fiber and as a sintering binder for porous ceramic articles.
However, difficulties are encountered in converting these polysilazanes readily and economically into the form of fibers or coatings that, after pyrolysis, yield ceramic products in the form of fibers, films, coatings of various gauges and molded articles. One attempt to solve this problem is provided in U.S. Pat. No. 3,853,567. This patent discloses a process that manufactures shaped articles, such as fibers, comprising silicon carbide, silicon nitride or mixtures thereof, and other ceramic products. The process carries out a first thermal treatment of a polysilazane at a temperature between 200.degree. C. and 800.degree. C. to obtain a meltable carbosilazane resin that can be melt-spun and then pyrolyzed at a temperature between 800.degree. C. and 2000.degree. C.
Although the process of this patent represents a significant step forward, it has two disadvantages. Namely, it requires a first thermal treatment at a temperature that is very high, such as 200.degree.-800.degree. C. The process also requires the carbosilazane to be in melt form under anhydrous conditions and in an inert atmosphere. The weight yield of ceramic from this process is typically inadequate.
Japanese Patent Application No. 77/160,446 describes a process for the polymerization of an organopolysilazane of high molecular weight by using acidic earths as a catalyst for the treatment of the organopolysilazane. However, in this process, the solid catalyst needs to be separated off by filtration. This separation involves the use of a solvent in the case of polymers that can reach high viscosities. Furthermore, the process of this Japanese Patent rules out the use of polysilazanes containing .ident.SiH groups but does not rule out the possibility of using polysilazanes containing unsaturated aliphatic hydrocarbon groups bonded to silicon atoms. These aliphatic hydrocarbon groups include alkenyl groups, for example vinyl or allyl.
U.S. Pat. No. 3,007,886 describes a process for the treatment of polysilazanes with metal salts of monocarboxylic acids that are soluble in hydrocarbon solvents. U.S. Pat. No. 3,187,030 describes a process for the treatment of polysilazanes with specific metal salts of strong inorganic acids. In this process, the catalytic activity is ensured by the metal cations that act as a Lewis acid.